1. Field of the Invention
The present invention relates generally to a copying machine, and more particularly, a copying machine including an improved sheet feeding device which allows the sheet to be fed synchronously with the speed of the original carrier. Herein, the sheet means any material on which the images (i.e. letters and/or pictures) in the original can be reproduced or copied.
2. Description of the Prior Art
In order to explain the background of the invention in detail, reference will be made to FIGS. 3 and 4 which illustrates a typical example disclosed in Japanese Patent Publication (unexamined) No. 59(1984)-48343.
Referring to FIGS. 3 and 4 the copying machine includes a motor M having a speed reducer, an original carrier 101 for carrying an original 102, an endless belt 103 running on a plurality of pulleys, wherein the original carrier 101 is fastened to the endless belt 103 by means Of a suitable fastener 104 so as to ensure their unitary movement, and a pair of feed rollers 105 and 105a adapted to feed the sheet 106 therebetween by friction. The images in the original 102 are reproduced on the sheet 106. The feed roller 105 have a diameter equal to that of an input pulley 105b of the feed roller 105, so as to enable them to rotate synchronously at the same circumferential speed.
The optical system includes a light source 107 for giving light to the original 102, a lens 108 and a mirror reflector 109. There are provided a roller 110 for stretching the sheet 106, and a cutter 111 for cutting the sheet 106 after the images on the original are reproduced thereon.
The driving motor M is reversibly rotatable, and when it is rotated in a clockwise direction, the original carrier 101 is moved in the direction (B) indicated by the arrow. When the original 102 has been scanned through a slit (not shown), the motor M is rotated in a counter-clockwise direction, thereby causing the original carrier 101 to move oppositely to the direction (B) until it returns to its starting position. When the original carrier is returning, the rollers 105 and 110 are prevented against their reverse rotation in the known manner, for example, by means of a one-way clutch.
The feed roller 105 is made of rubber so that it is prevented from slipping on the sheet 106 and the input pulley 105b is made of metal.
Metal and rubber have different coefficients of expansion, and when the feed roller 105 and the input pulley 105b are subjected to a rising temperature, their diametrical lengths become unequal due to their different coefficient of expansion. As a result, different circumferential speeds results. This causes a poor reproduction of the images in the original.
The inventor has investigated the cause, and ascertained that it is entirely due to the different coefficient of expansion of the rubber and metal constituting the feed roller 105 and input pulley 105b. As is well known, the ambient temperature around the copying machine tends to rise partly due to the heat emitted by the working machine, and also to the climatic factor. It is therefore unavoidable for the feed rollers 105 and input pulley 105b to be subjected to a rising temperature. As a result of the different circumferential speeds, the synchronism between the movements of the original carrier 101 and the sheet 106 is lost.
The following facts have been demonstrated by experiments:
When the temperature rises from 10.degree. C. to 35.degree. C., a shortage of 1.1 mm per 300 mm is caused on the length of the sheet 106 to be fed. This reduces the resolving power to about 7.5 lines/mm against 10 lines/mm in a normal state, where the scanning speed is in full synchronism with the feeding speed of the sheet. Particularly in the case of a copying machine for producing a direct plate used in the photomechanical process the resulting poor quality is fatal.